Among the various types of organopolysiloxanes already known in the art, MQ organopolysiloxane composed of the monofunctional siloxane unit (M unit) and tetrafunctional siloxane unit (Q unit) (refer to Japanese Patent Application Laid Open Number Sho 61-195129) is by virtue of its heat resistance used as a starting material for varnishes and pressure-sensitive adhesives. More recently, hydroxyphenyl-containing MQ organopolysiloxane has been taught by Japanese Patent Application Laid Open Number Hei 1-292036 and chloromethyl-containing MQ organopolysiloxane has been taught by Japanese Patent Application Laid Open Number Hei 2-153935. One of the present inventors has also already proposed MQ organopolysiloxane that contains epoxy-functional organic and alkoxysilylalkyl groups (Japanese Patent Application Laid Open Number Hei 5-105758) and MQ organopolysiloxane that contains epoxy-functional organic groups and diorganopolysiloxane residues (Japanese Patent Application Number Hei 3-331409). However, MQ organopolysiloxane having alkoxysilylalkyl and C.gtoreq.6 alkyl groups on the monofunctional siloxane unit (M unit) silicon has heretofore been unknown.